In U.S. Pat. No. 4,680,638, a method is taught for concealing defects in film by using an infrared signal with a video signal to control the substitution of information from other parts of the film in those defect regions. This patent notes that “cyan film dye has a slight effect on transmission of infra-red radiation” and that some “masking” is thus required. There is no detailed description of this masking process and no film model is used in relation to the defect detection process. The process described therein may produce infrared channels with some residual cross-channel interference from the other color records in the film. This will subsequently reduce the effectiveness of the detection and correction process. For this reason, a means of overcoming this limitation was desired.
In U.S. Pat. No. 5,266,805, a method is taught for compensating for defects on film recording media via the use of an infrared channel. The correction of obscured pixels is obtained using information contained in the frame of one image. The correction method allows for defects, such as dust and scratches, to be removed through the judicious application of spatial image processing of the image data. This patent discloses that crossover in the scanned red color record may be removed via linear algebraic methods (col. 11, lines 44-65). This method is an effective yet somewhat simplistic means of removing unwanted interactions between the dyes during the scanning process. Higher-order interactions and non-linear responses may occur during the film-development process resulting in image content being present in the detected infrared channel. And a non-iterative approach to solving for the cross-correlation can be sensitive to outliers. Because of the importance of obtaining a statistically independent infrared channel, alternative improved methods of decorrelating the input channels (red, green, blue infrared) are sought. Virtually all scanned film defect detection/correction methods require a high quality infrared channel in order to provide high quality detection/correction. So while the presently known method of decorrelation is often satisfactory for some film scans, improvements to the decorrelation process will yield much better detection of defects.